1. Field of the Invention
This invention relates to new solutions of isocyanato-isocyanurates in plasticizers for polyvinyl chloride, to a process for producing these solutions by trimerizing some of the isocyanate groups in the diisocyanates on which the isocyanato-isocyanurates are based in the presence of the plasticizers as solvents or with subsequent dissolution of the isocyanato-isocyanurates in these plasticizers, and to the use of the solutions as adhesion-improving additives for coating compositions based on plasticized polyvinyl chloride.
2. Description of the Prior Art
In the coating of organic substrates, particularly substrates containing isocyanate-reactive groups, with melts of flexible PVC or PVC plastisols, the bond strength of the coatings often fails to meet practical requirements. Because of this, so-called adhesion promoters are used for improving adhesion (cf. Beschichten mit Kunststoffen, Carl Hanser Verlag, Munich, 1967 and Kunststoffe 68 (1978), pages 735 et seq., pages 800 et seq.).
Adhesion promoters ideally suited to this purpose are, for example, the solutions of isocyanato-isocyanurates in plasticizers for PVC which are described in German Auslegeschrift No. 2,419,016.
However, these known adhesion promoters still do not entirely satisfy practical requirements. In particular, where the known adhesion promoters are used, undesirable discoloration often occurs at the temperatures at which the PVC plastisols have to be processed (140.degree. to 210.degree. C.). This discoloration is not observed in the case of products which are free from adhesion promoters. In addition, there is a need for one-component adhesion promoters with which it is possible to produce PVC plastisols containing adhesion-promoting additives which are characterized by an increased pot life by comparison with the plastisols containing known adhesion promoters.
Accordingly, an object of the present invention is to provide new adhesion promoters for PVC plastisols with which it is possible to produce PVC plastisols containing adhesion promoters and distinguished by an increased pot life and which, in addition, are substantially color-stable during the production of the coating at temperatures in the range of from about 140.degree. to 210.degree. C.
Surprisingly, this object is achieved by the solutions of isocyanato-isocyanurates according to the invention which are described in more detail in the following. This is surprising because neither the replacement of the tolylene diisocyanate preferably used in accordance with German Auslegeschrift No. 2,419,016 by the 4,4'-diisocyanato-diphenyl methane which is mentioned in column 2, line 60 of the above-mentioned prior publication, nor the trimerization of mixtures of diisocyanates and monoisocyanates which is also recommended in the prior publication is suitable for achieving the object in question. Isocyanato-isocyanurates based on 4,4'-diisocyanato-diphenyl methane show only limited solubility in the plasticizers normally used for PVC and often precipitate from the solutions in the form of crystals. In addition, the solutions of isocyanato-isocyanurates based on 4,4'-diisocyanato-diphenyl methane show very high viscosities in the usual processing concentrations, so that they are largely unsuitable for this reason as well. The trimerization of mixtures of monoisocyanates and diisocyanates which is recommended in German Auslegeschrift No. 2,419,016 gives isocyanato-isocyanurates characterized by a drastically reduced NCO-functionality which is reflected in poorer bond strength of the coatings.